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Highly stretchable electroluminescent skin for optical signaling and tactile sensing

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Science  04 Mar 2016:
Vol. 351, Issue 6277, pp. 1071-1074
DOI: 10.1126/science.aac5082
  • Fig. 1 HLEC.

    (A) Image of the HLEC conforming to the end of a pencil. (B) Exploded view of the HLEC showing its five-layer structure consisting of a ~1-mm-thick electroluminescent layer (ZnS-Ecoflex 00-30) that is sandwiched between two PAM-LiCl hydrogel electrodes and encapsulated in Ecoflex 00-30. (C) Stress-stretch curves of Ecoflex 00-30, the electroluminescent layer, and the composite device. The hydrogel data are shown in the inset because of its much lower elastic modulus.

  • Fig. 2 The capacitive and luminescent behavior of the HLEC display under uniaxial stretching.

    (A) A nominal electric field of ~25 kV cm−1 was applied to the HLEC at the start of the uniaxial test. Five lengths were measured using image analysis software to obtain λ1 across the width of the illuminated portion of the tensile bar. We report the mean and standard deviation of those measurements. At an engineering strain (grip to grip) of 395%, we measured the mean strain of the illuminated portion to be 487%, with a range of 420 to 549%. (B) The capacitance of the HLEC as a function of its uniaxial stretch (number of samples, n = 4). (C) The relative illuminance of the HLEC versus its uniaxial stretch (n = 4), plotted alongside predicted values (supplementary text).

  • Fig. 3 Multipixel electroluminescent displays fabricated via replica molding.

    The device measures 5 mm thick, with each of the 64 pixels measuring 4 mm. We show the devices in various states of deformation and illumination: (A) undeformed, (B) stretched, (C) wrapped around a finger, (D) folded, (E) rolled, (F to H) with subsets of pixels activated, and (I and J) subsets of pixels activated while being deformed.

  • Fig. 4 HLEC skins endow soft robots with the ability to sense their actuated state and environment and communicate optically.

    (A) Schematic of a three-chambered soft robot. A series of three independently actuated pneumatic chambers is embedded between the HLEC skin (top) and a strain-limiting layer (bottom). (B) Capacitance plotted versus the actuation amplitude, defined as the relative change in deflection between the uninflated and fully inflated states (number of samples, n = 5). (C) A firm finger press induces an ~25% increase in capacitance. (D) Change in capacitance versus applied pressure. We observed a negligible change in the capacitive response of the sensors over a period of 120 hours. (E) Array of three HLEC panels, each emitting a different wavelength through selective doping of the EL phosphor layer. Each HLEC panel is activated independently. (F) An undulating gait is produced by pressurizing the chambers in sequence along the length of the crawler. This sequence produces forward locomotion at a speed of ~4.8 m hour−1 (~32 body lengths hour−1). As each pneumatic chamber is pressurized, the outer electroluminescent skin is stretched, increasing the electric field across the EL layer and thus the luminescence.

Supplementary Materials

  • Highly stretchable electroluminescent skin for optical signaling and tactile sensing

    C. Larson, B. Peele, S. Li, S. Robinson, M. Totaro, L. Beccai, B. Mazzolai, R. Shepherd

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S7
    • Table S1
    • Reference (33)
    • Captions for Movies S1 to S3
    • Captions for Data Tables S1 to S11
    Data Tables S1 to S11
    Data S1: Capacitance vs. uniaxial stretch (Fig. 2B)
    Data S2: HLEC illuminance vs. uniaxial stretch (Fig. 2C)
    Data S3: Capacitance vs. biaxial stretch (Fig. S2)
    Data S4: Capacitance change from finger press (Fig. 4C)
    Data S5: Change in capacitance vs. applied pressure (Fig. 4D)
    Data S6: Capacitance vs. actuation amplitude (Fig. 4B)
    Data S7: Capacitance vs. actuation amplitude for various system states (Fig. S5)
    Data S8: Resistance vs. stretch (Fig. S1)
    Data S9: HLEC illuminance vs. applied voltage (Fig. S3)
    Data S10: Stress vs. stretch ratio for the HLEC device and its constituent layers (Fig. 1C)
    Data S11: Strain measurements of the illuminated HLEC vs. engineering strain (Fig. 2A)

    Images, Video, and Other Other Media

    Movie S1
    A hyperelastic light-emitting capacitor (HLEC) is uniaxially stretched to >480%. The HLEC consists of five layers: electroluminescent ZnS particles are embedded in a 1 mm thick silicone sheet with a hydrogel electrode on both sides and an outer layer of silicone to provide structure and electrically insulate the device. An AC voltage (2.5 kV at 700 Hz) is applied to the two electrodes, causing the ZnS particles to emit light. This HLEC consumes 0.2 W and has an illuminated region measuring 4 cm by 5 cm in the undeformed state.
    Movie S2
    A multi-pixel display with a grid of 64 HLECS is fabricated using replica molding. The silicone encapsulation electrically insulates the display so that it can be safely handled while operated at an AC voltage of 2.5 kV. The highly deformable display is stretched, rolled, folded and conformed to the tip of a finger. Each pixel measures 4 mm.
    Movie S3
    Three HLEC panels are integrated into a soft robot measuring 15 cm in length. The robot consists of a silicone body with three internal chambers. These pneumatic chambers are pressurized in sequence to create a crawling gait, moving the robot forward at a speed of ~32 body lengths hr-1. The thickness of the HLEC is decreased as each pneumatic chamber is pressurized, increasing the electric field across the electroluminescent layer. The increased electric field leads to increased illuminance as the inflated panels are 22 inflated. In addition to emitting light, the HLEC panels also serve as sensors for the soft robot. The capacitance of each panel changes as it is deformed. These changes in capacitance allow the robot to sense the actuated state of each pneumatic chamber. The HLEC is also sensitive to tactile inputs, allowing the robot to sense its environment.

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